It is important in streaming applications that audio is played at the same time that a video event creating the audio is rendered. Such synchronization of audio and video signals provides fewer distractions to a user than audio and video that is even a few (e.g. >30) milliseconds out of synchronization.
Traditionally, synchronization of streaming audio and video signals has been accomplished by time stamping audio and video frames at a sender. A receiver buffers the audio and video signals and renders them according to their time stamps. The time-stamped audio and video signals must be transmitted over a digital network, such as an IP (Internet Protocol) network or an ISDN (Integrated Services Digital Network), where the time stamps can be digitally embedded in the stream as a field. The digital network may not have quality of service (QoS), which may result in unreliable service.
In some situations, such as a video conferencing scenario, the audio signal may be transmitted over a different type of network than the video signal. For example, while the video may be sent via IP or an ISDN, the audio may be sent over a PSTN (Public Switching Telephone Network), which has QoS and is highly reliable and ubiquitous. However, such a network transmits analog signals and there is no simple method for embedding a time stamp in an analog signal.